1. Field of the Invention
This invention relates to methods and apparatus for estimating cylinder torque in a system including a multi-cylinder engine and, more particularly, to a method and apparatus for estimating such cylinder torque based on system angular response measurements and a mathematical model of the system.
2. Description of the Prior Art
The estimation of cylinder torque in a multi-cylinder engine provides insight into the performance and health of each cylinder in the engine. Several related prior art methods have been developed for estimating cylinder torque based on system angular response measurements and a mathematical model of the system. A survey of such prior art methods is provided by Jeremy Williams in "An Overview of Misfiring Diagnostic Techniques Based on Crankshaft Angular Velocity Measurements", SAE 960039, in Electronic Engine Controls 1996 SP-1149, SAE International Congress and Exposition, Detroit, Mich., Feb. 26-29, 1996. incorporated herein by reference.
In essence, all of the related prior art methods have the following elements in common:
1. Develop a mathematical model representing the angular dynamics of the system; PA1 2. Measure the angular response at one or more places in the system; and PA1 3. Use the measured angular response and the mathematical model to calculate the system inputs.
U.S. Pat. No. 4,843,870 to Citron et. al., incorporated herein by reference, uses measured crankshaft angular velocity and a system model to calculate the indicated torque waveform applied by the engine cylinders. Although the Citron et al. patent teaches a reconstruction of engine torque from the measurement of angular velocity, it fails to recognize that the crankshaft is a rather flexible body under certain conditions, particularly in multi-cylinder engines operating at medium and high engine speeds. Since the system model does not allow for crankshaft torsional deflection, the method is limited by the conditions under which the rigid crankshaft assumption is valid.
U.S. Pat. No. 5,771,482 to Rizzoni, incorporated herein by reference, uses measured crankshaft angular velocity and a system model to calculate the indicated torque waveform applied by the engine cylinders. Rizzoni teaches a first method in which the crankshaft is modeled as two lumped inertias connected by a flexible element. However, this method does not fully model the dynamic torsional flexibility in the crankshaft. Since the system model does not allow for crankshaft torsional deflection between each crank throw, the method has limited use under conditions in which dynamic deflection in the crankshaft between each cylinder is significant.
In a second method, Rizzoni models the crankshaft as fully flexible, with a flexible element between each crankshaft throw. In engines in which the number of cylinders is greater than the number of angular response measurement locations, the number of unknown system inputs is greater than the number of known outputs. In such a case, Rizzoni teaches using a least squares, pseudo-inverse technique to solve for the unknown cylinder torque inputs at a single frequency. However, since the equation set relating the known outputs to the unknown inputs is rank deficient, the pseudo-inverse approach taught by Rizzoni does not accurately estimate the individual cylinder inputs.